Fuels & Thermochemistry | Topic Notes

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Fuels & Thermochemistry | Topic Notes

Fuel and Heats of Reactions.

• Crude Oil is a fossil fuel and formed from the bodies of tiny sea creatures that died

millions of years ago, as the layers of mud and slit grew thicker over the years, the

decay caused by bacteria slowly turned these bodies into crude oil and natural

gas.

• It is separated into a number of useful mixtures through a process called Fractional

Distillation.

• This involves heating the crude

oil and separating the various

mixtures on the basis of their

boiling points, each one of

these useful mixtures is called a

fraction.

• The crude oil is heated in the furnace

and enters the fractionating column as

a partially vapourised mixture.

• The fractionating column is a

tall cylindrical tower containing a

series of trays to help collect

the condensed liquids.

• Maintained at high temperature

at the bottom and the temperature decreases up along the

column.

• The larger hydrocarbons with higher boiling points turn to liquids near the bottom

of the column.

• The smaller hydrocarbons remain as gases and rise up along the column.

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• Different fractions condense at different levels in the fractionating column.

• Real - Refinery gas, bottled for sale as domestic gas, fuel C1 - C4

• People - Petrol or light gasoline, motor fuel C5 - C10

• Never - Naphtha - petrochemical industry, medicines, plastics, solvents and

synthetic fibres C7 - C10

• Kick - Kerosene - aviation fuel and in stoves C10 - C14

• Dirty - diesel oil, fuel in trucks, buses, trains and some cars C14 - C19

• Little - Lubricating oil - reduces wear of engines C10 - C35

• F****** - Fuel Oil - ships, power stations and heating plants C30 - C40

• B****** - bitumen - road surfacing/tar, waterproofing and roofing. C35 <

Lubricating Oil, Bitumen and Fuel Oil are the residue fractions, these are the

fractions that are left over when the more volatile fractions boil off.

Mercaptens are sulfur compounds that are added to odourless gases such as

refinery gases so that gas leaks can be detected.

Octane Number is a measure of a fuels tendency to resist knocking or auto

ignition measured on a scale of 100 being assigned to 2,2,4 trimethylpentane

and 0 being assigned to heptane.

• The early explosion of the petrol air mixture in the car’s cylinder is auto ignition or

‘knocking’, the engine can lose power and the cylinders can be damaged as a

result, the pistons vibrate and a metallic noise is heard.

• The shorter the alkane chain the higher the ON rating

• The more branched the alkane chain, the higher the ON rating

• Cyclic compounds have a higher ON than straight chain compounds

Tetraethyl lead was used in 1920’s as an anti-knock additive, lead pollution from car

exhausts had the potential to cause health damage. This lead to ‘unleaded petrol’

Fuels & Thermochemistry | Topic Notes

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Lead compounds are toxic and poison metal catalysts in the catalytic converter

which means they can no longer reduce levels of pollutants in exhaust fumes.

Isomerisation

Involves changing straight chain alkanes into their isomers.

• The alkanes are heated in the presence of a suitable catalyst and this causes

chains to break. When they do they are allowed to rejoin together and as a result

they are more likely to become branched chain alkanes as opposed to straight

chain alkanes.

• The straight chain alkanes are then again recycled over a suitable catalyst that

converts them to branched chain alkanes

• Pentane and hexane

Catalytic Cracking

Involves the breaking down of long chain hydrocarbon molecules to short

chain hydrocarbon molecules for which there is a greater demand.

• Short chain hydrocarbons are used in petrol - huge demand, they tend to be highly

branched which also increases the octane number.

• Involves heating heavier fractions in the presence of a catalyst

• A common reaction is when an alkane is converted into an alkene and a branched

alkane. If you split a saturated alkane atleast one product is unsaturated.

• Alkenes are important feedstock for petrochemicals industry - polythene,

polypropene etc...

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Reforming or Dehydrocyclisation

Involves the use of catalysts to form ringed compounds.

• Straight chain alkanes are converted to cycloalkanes and then cycloalkanes are

converted to aromatic compounds. Petrol contains 3/4% benzene - high ON.

Adding Oxygenates

any fuel that contains oxygen in its molecules, eg. methanol, ethanol, MTBE

(methyl tertiary butyl ether).

Increases octane number AND gives rise to very little pollution. They are cleaner

fuels than hydrocarbons as there are less carbon monoxide in exhaust of cars

(Brazil, ethanol)

Hydrogen gas is another fuel. There are 2 methods of manufacturing it:

(I) Steam reforming of natural gas - CH4 + H20 = 3H2 +CO, reacting methane

with steam in presence of suitable catalyst.

(II) Electrolysis of water - H20 -------> H2 +0.502, more expensive because of

high electricity costs

Burning hydrogen is very environmentally friendly - only by product is water, used to

manufacture ammonia, hydrogenating vegetable oils in making margarine.

hydrogen forms an explosive mixture with air so there are problems with storage

and transport.

Fuels & Thermochemistry | Topic Notes

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Thermochemistry

Exothermic - any chemical reaction that produces heat is exothermic and is represented by -

delta H. MINUS DELTA H. e.g. burning of food inside the body, the reaction of sodium dichromate

and ethanol.

Endothermic - a reaction that takes in heat - + Delta H. e.g. sherbert with water

Heat of reaction is the heat change that takes place when the number of moles of reactants

indicated in the balanced equation for the reaction react completely.

Thermochemical equations must be balanced, the value of delta H must be written and the

states of reactants and products must be given, e.g. gas, liquid etc...

Heat of combustion is the heat change that takes place when 1 mole of a substance is

completely burned in excess oxygen.

(general term)

important notes:

• C + 1/2O2 --------> CO delta H = -111kJmol^-1 This is not the heat of combustion because when

carbon is COMPLETELY burned in excess oxygen it forms carbon DIOXIDE not carbon

monoxide.

• Heat of combustion is measured using a bomb calorimeter.

• Small steel container with a screw on cap, the sample is placed in the crucible, the bomb is

placed in the container of water (calorimeter), oxygen is pumped into the bomb and the sample

is ignited by the electric wires.

• Bomb calorimeters also compare the efficiency of various fuels and gives the kilogram calorific

value of substances with a variable composition (coal, petrol, peat).

The kilogram calorific value of a fuel is the heat energy produced when 1kg of the fuel is

completely burned in oxygen.

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Bond energy is the energy required to break one mole of covalent bonds and to separate

the neutral atoms completely from each other.

Heat of neutralisation is the heat change that takes place when 1 mole of H+ ions from an

acid react with 1 mole of OH- ions from a base.

Heat liberated = M X C X (T1-T2)

C = specific heat capacity.

REMEBER MASS IS IN KILOGRAMS

• When 200 cm^3 NaOH solution was added to 200 cm^3 of a 0.4M sulfuric acid solution, a

neutral solution was produced and temperature increased by 5.5 degrees.

• C of neutral solution = 4.2 kJ kg^-1 degrees celsius to the -1.

• Density is 1g/cm^-3

C =4.2 X 1000= 4,200 Joules

Plastic container is used because plastic has a NEGLIGIBLE specific heat capacity, i.e. we can

assume the container itself doesn’t absorb heat.

Mass of the solution = 1 gram per cm^3 density X (200+200)cm^3 = 400g = 0.4KG

Heat liberated = M X C X temp rise = 0.4 X 4,200 X 5.5 = 9,240 Joules

Number of moles of H2SO4 Liberated = (volume X Molarity) divided by 1000 = (200 X 0.4)/ 1000 =

0.08 Moles

0.08 Moles of H2SO4 Liberates 9240 joules

1 mole liberates 9240/0.08 joules/ 115,000 joules

115,000 joules is 115.5 KJ

H2SO4 + 2NaOH ------> Na2SO4 + H20

i.e. 2H+ + 2OH- -----> 2H2O

Heat of neutralisation is when 1 mole of H+ Ions react with 1 mole OH- ions, here there are 2

moles.

115.5/2 = -57.75 KJ molˆ-1

Heat of neutralisation is half the heat of reaction here

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The heat of formation of a compound is the heat change that takes place when one mole of

a compound in its standard state is formed from its elements in their standard states.

Standard state is at 25 degrees and at 1 atmospheric pressure or 101 KPa

2C + H2 ------> C2H2 deltaH= 233kJmol^-1 you will be required to write down heats of formation

and sometimes fractions will be necessary.

1/2N2 + 3/2H2 -----> NH3

Hess’s Law states that if a chemical reaction takes place in a number of stages, the sum of

the heat changes in the separate stages is equal to the heat change if the reaction is

carried out in 1 stage.

A -----> B ------> C -----> D

Delta H of A --> D = deltaH of A-->B + DeltaH of B-->C + DeltaH of C--->D

May be used indirectly to measure the heat of reaction of a chemical reaction that may be difficult

to measure.

1. Calculate heat of formation of a compound using other

heats of formation and one heat of reaction

(i) C + O2 ----> CO2 deltah= -393

(ii) H2 + 1/2O2 --> H20 deltah=-286

(iii) CH4 + 2O2 --> CO2 + H20 deltah=-879

calculate the heat of formation of methane.

what we need: C + 2H2 ----> CH4 deltaH= ?????

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Start by introducing C - C + O2 ---> CO2 deltah=-393

introduce 2H2 by multiplying (ii) by 2 : 2H2 + O2 ---> 2H20 deltah= -572

add both equations here

C + 2H2 +202 ----> CO2 + 2H20 deltah = -393-572 = -965

We need CH4 on right hand side so REVERSE equation (iii) and add to the previous

equation:

reversed - CO2 + 2H20 ----> CH4 + 2O2 deltah= +879

Add - CO2 + 2H2 + 202 +2H20 + C ---> CH4 + 2H20 + CO2 + 202

this leaves us with

C + 2H2 ----> CH4 deltaH = -965 + 879 = -86

2. Calculate Heat of reaction using heats of formation of

reactants and products.

This is the same as above except a more complex equation IS GIVEN to us

with heats of formation of its elements.

Fuels & Thermochemistry | Topic Notes

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Sources of Hydrocarbons

Hydrocarbons

• compounds consisting of hydrogen and carbon only, bonded together covalently.

• important as fuels and feedstock for the chemical industry.

Coal, Natural Gas and Petroleum

• sources of hydrocarbons

• formed from the compacted remains of dead plants of animals which have been

fossilised

• crude oil and natural gas = marine animals and plants

Decomposition as Methane Sources

• anaerobic decay - animal waste, vegetation, rubbish dumps

Hazards of Methane Production

• extremely explosive

Structure of Aliphatic Hydrocarbons

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Aliphatic

• hydrocarbons which do not contain a benzene ring

Homologous Series

• a homologous series is a family of organic compounds with the same general formula,

similar chemical properties, gradations in physical properties, and each member differs

by a CH2 unit

No. of Carbons Root

1 Meth

2 Eth

3 Prop

4 But

5 Pent

6 Hex

7 Hept

8 Oct

9 Non

10 Dec

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Alkanes - names, structural formulas, isomers (to C-5)

• -ane

• CnHn+2

• saturated (only single bonds)

• making isomers: shorten chain by one, add methyl group to an inner carbon

• tetrahedral shape

Alkanes - physical properties

• lower (up to butane) - gases @ rt

• middle (up to C15) - liquids @ rt

• high (C15+) - waxy solids @ rt

• longer chains ➝ stronger van der Waals’ ➝ higher boiling point

• non-polar - cyclohexane and methylbenzene

Alkenes - names, formulas, isomers to C-4

• CNH2N

• C = C (unsaturated)

• lowest member is ethene

• isomers of butene: but-1-ene, but-2-ene, 2-methylpropene

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• lower - gases,

• higher - liquids or solids

• non-polar or slight polarity

• longer chains ➝ stronger van der Waals’ ➝ higher boiling point

Prep and Properties of Ethene

• ethanol is dehydrated using hot aluminium oxide (white powder)

• non-polar so collected by displacement of water

• C2H5OH ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⟶ C2H4 + H2O

•

•

(III) combustion: sooty, smoky flame. Limewater ⟶ milky

(IV) unsaturation: bromine water ⟶ cloudy

(V) unsaturation: acidified potassium manganate(VII) ⟶ cloudy

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Alkynes - Ethyne / Acetylene

(VI) C2H2N-2

(VII) C≣C (unsaturated)

(VIII) lowest member is ethyne

(IX) longer chains ➝ stronger van der Waals’ ➝ higher boiling point

(X) lower - gases,

(XI) higher - liquids or solids

(XII) non-polar or slight polarity

(XIII) uses: cutting metal and welding - oxyacetylene torch

Prep and Properties of Ethyne

(XIV) impurities: calcium sulfide & calcium phosphide ⟶ phosphene, hydrogen sulfide

(XV) acidified copper sulfate soln to remove impurities

(XVI) calcium dicarbide & water

(XVII) CaC2 + 2H2O ⟶ C2H2 + Ca(OH)2

2.

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(iv) liquid in dropping funnel = water

(v) X = calcium dicarbide

(vi) Y = acidified copper sulfate soln

(vii) colourless, sweet smell

(viii) combustion: explosive, basic

(ix) unsaturation: bromine water

(x) unsaturation: potassium manganate(VII)

Aromatic Hydrocarbons

Structure of Benzene, Methylbenzene and Ethylbenzene

(xi) carbon-carbon bonds are intermediate between single and double bonds

(xii) (due to) delocalised pi electron cloud

(xiii) note: benzene is carcinogenic

3.

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4.

Uses of Aromatic Compounds

• methyl benzene - industrial solvent - dissolves non-polar solvents

• martius yellow - dye

• folic acid - pharmaceuticals

• morphine - pain killer

• diuron - herbicide

• phenolphthalein & methyl orange - acid-base indicators

• aspirin - pain killer (non-carcinogenic)

Physical Properties

• non-polar

• lower - liquids

• higher - solids

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